Prior to the introduction of solid state electronics, the operating temperature of electronic devices was not of much consequence. This was mainly because electronic parts such as capacitors, inductors and resistors were made of metallic materials such as copper or silver. These metallic materials did not experience unacceptable conductive property changes over wide temperature ranges. Thus, the electrical components could be exposed to a wide range of temperature environments without concerns of changes in the components' performance.
With the advent of solid state devices, however, device operating temperatures became a concern. The advantage of solid state devices is that their conductivity ranges between the conductivity of metals and insulators. In practice, the conductivity of a solid state device such as silicon is modifiable by doping the silicon with impurities, thereby changing the devices characteristics to a desired conductivity. The problem, however, is that the conductivity of solid state devices is also changed due to device operating temperature. For this reason, many electronic devices on the market today will only operate with the desired characteristics within certain temperature ranges. Although many electronic applications operate within these acceptable ranges, concerns are raised when operating temperatures are at either the lowest or highest limits of these ranges.
For many years, designers have placed emphasis on heat dissipation. Many electronic applications operate in either room temperature or higher temperature environments and thus require techniques for heat removal to keep the electrical components at acceptable operating temperatures. In fact, heat dissipation has become an extremely important concern in the area of electronic assembly miniaturization. In these applications, the close, physical proximity of electronic devices produces heat dissipation problems which must be remedied. To combat these heating problems, the industry has created many techniques such as the use of heat sinks and micro-electromechanical devices (i.e., fans and pumps) to facilitate heat removal.
Although there has been many strides in effective heat removal for electronic devices at room temperature or higher temperature ranges, very little has been done in the industry to heat devices which operate in lower temperature ranges. However, there is a need for such techniques. For example, electronics used in aircraft and space applications are subjected to extremely cold temperatures associated with the upper atmosphere and space. Further, electronics are also used in deep oceanography (i.e., submarines) and in arctic conditions. Finally, electronics are utilized in everyday refrigeration and air cooling units and also in winter specific applications. These applications operate at low temperatures which may cause the electrical components to either fail or operate in an undesirable manner.
In the past, designers who are faced with applications that will be subjected to lower temperatures have used specifically designed electrical components which operate with desired properties at lower temperatures. These devices are quite expensive and sometimes do not operate with desired characteristics in extremely cold temperature conditions. Because of the expense and unreliability of these specifically designed electrical components, there is a need for heating techniques which would allow the use of common commercial electronic devices.
The aircraft and aerospace industry are accustomed to dealing with the problems occasioned by low temperature affects on air and space craft equipment. For instance, many techniques have been developed to combat icing on wings and engine parts during flights. In particular, U.S. Pat. No. 4,743,740, to Adee and U.S. Pat. No. 5,475,204 to Giamati et al. provide techniques for heating the wing and engine areas on airplanes. These techniques utilize metallic, resistive elements which are connected to a power source so as to produce thermal energy as current is passed therethrough.
Although these techniques provide heat in low temperature applications, these techniques would be difficult, if not impossible, to apply to the heating of electrical components. For instance, these heating devices are embedded into parts where there is little concern for size of the conductors. In contrast, however, overall structural size is a major concern in the electronics industry. Second, these heating techniques tend to produce non-uniform heating as disclosed in '204 to Giamati et al. Non-uniform heating may cause particular problems for electrical components because some portions of the component will be operating at a satisfactory temperature while other portions are not.
Finally, these prior techniques do not address the problems with electronic noise generated by most heater designs. In fact, these heating devices often include straight parallel conductors which may produce undesirable electronic interference. For example, electromagnetic or magnetoelectric energy generated by one or more electrical components constitutes electronic noise which may couple to the straight parallel conductors of the heater through ordinary magnetoinductive or radiant energy coupling phenomena. This noise can propagate through the heating device and, in turn, reverse couple into other electrical components, thereby causing undesired electronic interference.
At least one company currently produces a heating element specifically designed for the heating of electrical components. The Minco Company of Minneapolis, Minn. manufactures a Thermofoil brand heating device. This heater is composed of electrical heater strips that employ wide traces of alloyed, electrically conductive materials plated upon an electrically insulated substrate, such as Mylar, to form a electrically resistive heating element. Unfortunately, these Thermofoil heating devices also suffer from several deficiencies. For instance, these Thermofoil heaters include parallel traces which do not provide for uniform heating. Second, the parallel traces also can cause electronic interferences as previously discussed. Third, these heaters are usually attached to the top of electrical components, opposite the circuit board to which the electrical component is mounted. This configuration is inefficient because some of the heat is dissipated to the air while the heat supplied to the electrical component must penetrate the casing of the electrical component. Finally, Thermofoil heaters are expensive to produce, costing about thirty dollars per unit. This cost may sometimes exceed the cost of specifically electrical components designed for colder temperatures, thereby making the heater not cost effective.
For the foregoing reasons, there exists a need for a heating device which provides uniform heating of electrical components, without producing significant electronic noise, at a reasonable cost.